The present invention relates to a production method for improving the mechanical properties of glass bottles.
Methods of producing glass bottles are classified into two types, the blow and blow process (B-B process), and the press and blow process (P-B process). Recently, P-B process, which is capable of producing glass bottles having a uniform distribution of glass material, has become employed in the production of all kinds of bottles and has become an important method of producing lightweight bottles. Production of lightweight bottles by P-B process, however, is likely to lower the impact strength of said bottles to the extent that the glass bottles on the conveyor, such as in the usual filling line, crack or break, causing various troubles.
For this reason, a method has been employed of sorting out glass bottles by a squeeze tester to discard glass bottles of low impact strength in advance from the viewpoint that the impact strength of trouble-prone glass bottles is generally in the neighborhood of 1 Kg-cm, although depending on the bottle weight and on the conveyor speed. This method, however, does not increase the percent pack of lightweight bottles.
On the other hand, an effort has been made to increase the impact strength by applying a chemical treatment to the inner and outer surfaces of glass bottles produced by P-B process, but this approach has not been successful in providing the sufficient impact strength.
My careful study on the problem of decrease of the impact strength of glass bottles produced by P-B process has revealed that foreign matter adhering to the plunger fitted in the blank mold is a cause of the degradation of mechanical properties. More particularly, a form of graphite consisting mainly of carbon powder which forms when an oily mold release agent consisting mainly of graphite applied to the bottle forming mold (blank mold, neck ring mold) is burnt at high temperature, and other various dusts including glass powder and iron powder floating in the atmosphere around the bottle forming machine adhere to the plunger, thus scratching the inner surface of the glass bottle during the formation of a parison or adhering to the inner surface of the bottle, and then entering the glass material to exert an adverse influence on the mechanical properties of the glass bottle. With this in mind, I have found that the strength, particularly impact strength of glass bottles, can be increased by preventing foreign matter from adhering to the plunger.